1. Field of the Invention
The present invention relates, generally, to an image sensor, and, more particularly, to an image sensor and a pixel circuit thereof that has a wide dynamic range due to logarithmic-linear-logarithmic operation.
2. Description of the Prior Art
Complementary metal-oxide-semiconductor (CMOS) image sensors are widely used in a variety of commercial applications. The major advantages of CMOS image sensors are high level of integration, low voltage operation and low production cost. However, CMOS image sensors usually have poor dynamic range when compared to a charge-coupled device (CCD) image sensors.
There are several ways to improve the dynamic range of a CMOS image sensor, one of which is logarithmic pixel. It is widely used in a three-transistor (3T) Photodiode pixel for some applications. Generally, an active or passive pixel responds linearly to the light intensity that enters the pixel. A logarithmic pixel is based on the sub-threshold operation mode of a MOS transistor to provide a logarithmic number of electrons into the photodiode when the photocurrent is so small that the transistor enters the sub-threshold region. However, such operation leads to following disadvantages:                1. Low photosensitivity especially in the low illumination region;        2. Slow response due to sub-threshold operation; and        3. A relatively large variation of the device characteristics due to sub-threshold operation.        
Therefore, there is still a need to provide a pixel circuit and an image sensor that can overcome the above-mentioned disadvantages.